In the culturing of cells, an ability to start observation simultaneously with occurrence of a cell mass, which is a mass of a plurality of cells, and then continue the observation along the time axis can be said to be a promising technology that may assist, for example, regenerative medicine. Conventionally, such observation of cells is performed under a microscope during the culturing of the cells when the culture container needs to be replenished with the culture fluid or the culture fluid needs to be replaced, and images (photos) are taken as necessary.
However, cell observation using a microscope requires much trouble. For example, to identify cell masses that have occurred inside a container, it is necessary first to observe the entire container with the eye or through a microscope, and then, after switching to a magnified scale as by interchanging objective lenses, to proceed to observation of the growing condition of individual cell masses. Magnified observation suffers from a narrow view field, and this makes it difficult to find target cell masses and also to bring them into the view field. Moreover, in cell observation, time lapse observation is desired so that long-term change of cell masses can be observed every predetermined period from their occurrence till completion of their growth. Immediately after semination of cells, it is impossible to observe cell masses with the eye or through a low-magnification microscope, and this makes it necessary to search for them and set the observation positions anew after several days.
Moreover, with conventional observation, which is performed about once a day to three days when the culture container is replenished with the culture fluid or the culture fluid is replaced, it is difficult to observe cell masses from their occurrence, and thus there are strong demands for technologies that allow observation of cell masses from their occurrence. Furthermore, both in observation of an entire container and in observation of part of the inside of a container on a magnified scale, when images of cells are taken, inconveniently, the heat generated by an illuminator and a lens drive system affects the growth of the cells.
For such cell observation, there have been proposed devices that eliminate the trouble of switching between observation of an entire container and observation of part of the inside of a container on a magnified scale, and one example of such devices is seen in Patent Document 1 listed below. The observation device disclosed in Patent Document 1 is provided with at least two image sensing optical systems that take images of an observation target at different magnifications, and the reference value for the low-magnification image is calculated such that the feature quantity of the image taken through the high-magnification image sensing optical system is approximately equal to the feature quantity of the image taken simultaneously through the low-magnification image sensing optical system.